In a typical communication network, wireless communication devices, also known as wireless devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio Access Network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into service areas or cell areas, with each service area or cell area being served by a radio node such as a radio access node e.g., a Wi-Fi access point (AP) or a base station (BS), which in some networks may also be denoted, for example, a “NodeB” or “eNodeB”. The area or cell area is a geographical area where radio coverage is provided by the radio node. The radio node communicates over an air interface operating on radio frequencies with wireless communication devices within range of the radio node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS), also known as Fourth Generation (4G) network, have been completed within the 3GPP and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as a radio access network of a Long Term Evolution (LTE) network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a 3GPP radio access network wherein the radio nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the radio nodes, this interface being denoted the X2 interface.
The ongoing 3GPP Rel-13 study item “Licensed-Assisted Access” (LAA) intends to allow LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, wireless communication devices connect in the licensed spectrum, via a primary cell or PCell, and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum, via a secondary cell or SCell. To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
Due to the LBT procedure, the first slot in which the LAA SCell or LAA UE is permitted to transmit cannot be predicted in advance. This makes it difficult to pre-compute the data payload since several parameters are currently dependent on the slot number in which data is transmitted.
Long Term Evolution (LTE)
LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM, also referred to as single-carrier (SC)—Frequency Division Multiple Access (FDMA), in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink subframe and the same number of SC-FDMA symbols in the time domain as number of OFDM symbols in the downlink subframe.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 ρs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signalling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Rel-11 onwards, above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in FIG. 3 are the cell specific reference symbols (CRS) that are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
The generation of the baseband transmit signal on the physical shared channels for either the downlink (DL) or uplink (UL) generally involve scrambling, modulation mapping, layer mapping, precoding, and RE mapping. The specific baseband chain for the UL Physical Uplink Shared Channel (PUSCH) is shown in FIG. 4 as an example. For PUSCH scrambling, the initialization of the scrambling sequence generator at the start of each subframe is a function of the current slot number ns. This is also true for Physical Downlink Shared Channel (PDSCH) scrambling on the DL.
Carrier Aggregation
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 wireless communication device. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable wireless communication devices compared to many LTE legacy wireless communication devices. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy wireless communication devices, i.e. that it is possible to implement carriers where legacy wireless communication devices can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 wireless communication device can receive multiple CC, where the CC have, or at least have the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 5. A CA-capable wireless communication device is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case when the number of CCs in downlink and uplink is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a wireless communication device: A wireless communication device may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of carrier aggregation is the ability to perform cross-carrier scheduling. This mechanism allows a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a wireless communication device expects to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling; this mapping from (E)PDCCH to PDSCH is also configured semi-statically. Licensed-assisted access (LAA) to unlicensed spectrum using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects that the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 6, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. A secondary cell in the unlicensed spectrum is herein denoted as licensed-assisted access secondary cell (LAA SCell).
Hybrid Automatic Repeat Request (HARQ) is a mechanism used in LTE to handle retransmission of missing or erroneous transmitted packets. The HARQ procedure consists in providing feedback, such as Acknowledgement (ACK) and Non-Acknowledgement (NACK), to the transmitter on a transport block basis, thereby offering the possibility to successfully decode a transport block very quickly. The probability to eventually decode with success a given packet is enforced by the soft combining technique that enforces the HARQ operation. In particular, a receiver implementing the soft combining scheme stores the erroneously received packet and later combines it with the retransmitted replicas of that packet requested by the HARQ feedback. Such replicas contain the same data as the original transport block but a different set of coded bits obtained with different redundancy versions, i.e. by using a different puncturing pattern of the code.
In legacy LTE, the uplink HARQ feedback, such as ACK and NACK, is conveyed by a Physical Hybrid-ARQ Indicator Channel (PHICH) that is transmitted by the radio node upon detection of an uplink transmission on the Physical Uplink Shared Channel (PUSCH) from the wireless communication device.
In legacy LTE, the downlink HARQ feedbacks, such as ACK and NACK, are conveyed by the Physical Uplink Control Channel (PUCCH). It is transmitted by the wireless communication device upon detection of a downlink transmission on the Physical Downlink Shared Channel (PDSCH) by the radio node. The wireless communication device determines to retransmit previous data when NACK is received or if no feedback is received.